Acoustic tensiometer



May 19, 1936. N PACK 2,04Q,874

ACOUSTIC TENS IOMETER Filed March 27, 1954 5 Sheets-Sheet 1 INVENTOR MEN ,EL. N. C

May 19, 1936.

M. N. PACK ACOUSTIC TENSIOMETER Filed March 27,

1954 3 Sheets-Sheet 2 y 9, 1936. M. N. PACK 2,040,874

ACOUSTIC TENS IOMETER Filed March 27, 1934 3 Sheets-Sheet 3 01/7 PUT METER OJC/LLOGRAPH I N V FIN TOR 59 l fifjvjgs-L /V. 940% di ij 4 T PA/EYS Patented ay 19. 1 I

UNITED STATES PATENT OFFICE amp-:4 ACOUSTIC rsusroma'rsa Mendel N. Pack, Dayton, Ohio Application March 27, 1934, Serial No. 717,002

23 Claims. (01. 175-183) (Granted 'under'the act of March 3, 1883, as

amended April 30, 1928; 370 0. G. 757) The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment to -me of any royalty thereon.

The present invention relates generally to measuring instruments, and more particularly to instruments for measuring the stressin a member under tension, such for example, as wires, propellers or any other object or objects having mass and elasticity.

In providing an instrument for determining the stress in a member under tension, advantage is taken of'the fact that an elastic body or mass under tension when plucked or excited into vibration and subsequently allowed to vibrate in a magnetic field, generates an alternating E. M. F, the frequency of whichis the natural frequency of the vibration of the body. By utilizing this alternating E. M. F., it is possible to obtain an audible note or signal in a pair of earphones or other sensitive reproducers capable of being compared with a musical note of known frequency in determining the frequency of vibration of the The primary object of the present invention is to provide in an instrument of this character, means for acoustically comparing the natural I frequency of vibration of any given length of a vibratory section with a standard of known frequency, in order to determine the frequency of vibration of the vibratory structure.

I A further object of the present invention is to provide in an instrument of this character, means for determining the stress in a member under tension by exciting said member to cause it to' vibrate in its natural frequency of vibration, and

embodying means for translating the vibration of said member into a musical note made audible 1 through the use of ear phone's, means for de- 40 veloping .a second musical'note' of a known frequency for comparison with said first mentioned note and further means under manual control for changing the natural frequencyiof vibration of said vibratory section to bring its note into resonance withthe note of known frequency. A still further object oftthe'present invention is to provide a measuring instrument capable of being used as a tensiometer' for tie rods oliother members having mass and elasticity, means being provided in the instrument for reading directly the stress-in, the member being tested without resorting to the use of tables or charts or, requir ing mathematical calculation.

A further'object of the present invention is to provide an instrument capable of measuring the Fig.3 is an enlarged detail top view of one of f ment. r

v.cuit and audio amplifying circuit used in connec- .be vibrated through large amplitudes, are eliml0 inated. Thus, in the case of tie-rods as used on aircraft, the anchorages for the rod and fairing used in connection therewith need not be removed in determining its stress.

A further object of the present invention is 15 to provide in an instrument of this character, calibrated means for indicating directlythe tension in members of different sizes and shapes, said means comprising a support of light weight material, a fixed clamping member and a'mova- 20 ble, manually controlled clamping member for clamping therebetween different portions of the length of the member being tested, the movable clamping member being provided with a pointer capable of being adjusted to register on any one 25 of a plurality of scales provided on said support.

With these and other objects in viewwhich will appear as the nature and advantages of the invention is better disclosed, the invention consists in the features of construction, combination 30 and arrangement of parts hereinafter fully described and claimed,,reference being had to the accompanying drawings in which: V

Fig. 1 is an assembled view of my invention illustrating the manner of determining the stress in the brace wires of the wings of an aircraft.

Fig.2 is an enlarged. detail ,view of the support with clamping members attached for clamping therebetweenpredetermined lengths of the wire tobe tested. 40

the clamping members.

Fig. 4 is a detailed view of the tuning instru- Fig. 5 is a pattern illustrating the arrangement of-scales on the clamp support for different gage wire. V N

Fig. 6 is a sectional view taken on the line 6-6 'of Fig. 2. v

' Fig. 7 is adetail of theadapter 'usedin con- 50 nection with my invention when the member being tested is of non-magneticmaterial'.

/ Fig.8,is a wiring diagram of the oscillator 'cir tion with my invention. 56

, Fig. 9 is a wiring diagram similar to that shown in connection with Fig. 8 and including an output meter for visually indicating when the produced notes are in resonance.

" Fig. 10 is a wiring diagram similar to that shown in Fig. 8 and including an oscillograph for producing permanent records of the produced notes.

Referring more particularly to the drawings wherein corresponding partsare, designated by like numerals throughout the several views thereof, the instrument in one embodiment ofmy invention comprises essentially a clamping device l0, electromagnetic pickup II and tuning instrument generally indicated by the numeral i2.

As shown more particularly in Fig. 2 the clamping device comprises essentially a supporting element if of light weight tubular materlal which is provided at its lower end with a primary clamping member II and at its upper end with a secondary clampingmember IS. The primary and secondary clamping members are each formed with split body portions l6 and I1 respectively to permit sliding of the same on the supporting element l3. Each body portion is further provided with suitable means as indicated by the numerals l8 and IQ for clamping the respective'body por-': tions of the clamping members in desired position upon the supporting element It.

The primary clamping member, as shown in Fig. 2, has formed integral therewith an extension v carrying a fixed inner clamping jaw 2| and an outer clamping jaw 22, adjustable toward and away from the fixed clamping jaw, through the medium of an adjusting screw 23. In this manner, as shown in Fig. 2, the wire to be tested, indicatedby the letter "a, may be readily clamped between the jaws of the lower clamping member. I I

The upper clamping member I, as shown more particularly in Fig. 3, has also-formed integral therewith an extension 24 carrying e fixed inner clamping jaw 25 provided with a roller 20 and an'outer clamping jaw 21, adjustable toward and away from the fixed inner clamping jaw through v the medium of an adjusting screw II. The outer clamping jaw, as will be noted, is also provided with a roller 29 mounted upon a transverse shaft 30 carried in the outerend of the clamping jaw.

, The roller 26 is pinned or otherwise similarly secured to a transverse shaft 3|, journaled in.

' bearing portions}! and 33 of the extension 24, the outer-end of the shaft 3| having fixedly;

mounted thereon a hand wheel 34, the purpose of which will be'hereinafter more fully set forth.

As'will be noted the lower end of the .body

portion H of the upper clamping member]! is provided'with a pointer 35 provided with a split;

collar portion 38 adapted tape sprung over the shouldered lower end '31: of theibodv'portijon J1; as shown in Fig. 6. By reason of the construc- 'tion it will be readily apparent that by rotation of the collar-portiondmthe pointer 36 maybe caused to register on eny oneof the s'everalscales 3| calibrated for'wires ofikn'owdstandard provided upon the periphery of the supportingielement l3. As shown in Fig. 5, the scales ll are calibrated for wires of diflerent gage. The numher of different sizes and shapes of wires which may be tested with one instrument is not limited to the number of calibrated scales which can be crowded-on the supporting element 13, inasmuch 'as there is also provided a length scale. This scale may be used in conjunction with calibration icurves 'for any number wiru of the lines 01' which are not included in the range of the scalev of standard.

Carried upon the supporting element ll intermediate the clamping members 14 and I'I,there is provided an electromagnetic pickup, generally indicated by the numeral II. This pickup comprises essentially a conventional permanent horseshoe magnet ll fixedly secured in a predetermined position on the supporting element It by means of suitable spacer blocks 42 of noning member. "Thus it will be apparent that the a length of wire intermediate the clamping jaws of the upper and lower clamping members respectively will be free to vibrate in the magnetic field of the magnet. The primary winding 43 and the secondary winding, is wound in coil form about the one pole of the magnet as shown in Figs. 2 and 8, and adapted for connection with the tuning lnstmment indicated by the numeral l2 and for this purpose the ends of the primary and secondary coils are electrically connected to a cable 45 provided atits free end with a suitable adapter 48. a

' The tuning instrument l2 may. for convenience, be carried in a portable case generally indicated by the numeral" having mounted therein a panel 40 for mounting thereon in suitable fashion the electrical elements necessary to. the

proper functioning. of. the instrument .as one adapted for measuring the stress in a member under tension.

8 illustrates diagrammatically the man- 1 ner in which the various elements are electrically connected to accomplish this end. As shown therein the adapter, which comprises mentially a four-prong base plug of conventional type is adapted for electrical connection witha fourprong base socket 40. In this'manner, from an observation of Fig. 8, it will benoted that the ends of the primary winding; 48 are-electrically connected to the input end of 'a conventional audio amplifying: circuit generally indicated by the numeral I. and comprising a plurality of amplifying tubes Ii, and audio transformers 52 for effecting an amplification of thecurrent received atthe' input end of the circuit. From the description thus far it will. be apparent that if the wire "a",'as, shown in Fig. 2, is plucked or struck by some blunt-instrument, as, for example,

subsequently allowed to vibrate in the steady a rubber mallet, and excited into vibration and magnetic, field which exists between the north 1. i

which through the medium of the audio ampliv fying circuit ll is translated into a musical note in a pair of telephone receivers II, as shown in Fig. 1, that may be connected to. conventional telephone jacks N secured thecircuit.

at the output end of Totransmit certain frequencies, i.*e.,

frequencieswithin the range. of frequency of vi- ,bration ofrthe wire bcingvtested, andsuppress all other frequencies of then. M1. generated. a fixed condenser ll of suitable capacity is-shunted acrosstheinput and of the circuitas shownin Pig. 8. In this manner undesired high frequen' cies are suppressed and frequencies in the desired range are transmitted into the audio circuit. I'hereis also provided an amplifier volume control 58, which consists essentially of'a potentiometer shunted across the input end of the circuit as shown in Fig. 8.

The tuning instrument also comprises an os cillatory circuit generally indicatedby'the numeral 51 which includes an audio oscillator II, the output from the plate and. grldcf which circuit is connected through the transformer 59 and fixed condensers", BI and '82 to thewires .2

generated in the oscillating circuit and fed back into the audio amplifying circuit II and translated into a second musical note. The frequency of the' noteproduced in the oscillatory circuit may be controlled through the medium of the. Thus a second musical"- note'of known frequency is obtained for comparison with the note produced'in the detector' circuit heretofore mentioned. A; cover." is, f. provided for the case 41, provided with'a. com-. partment within which'may be conveniently mounted the necessary batteries for operating variable condenser 83.

the circuits hereinabove set forth and as shown in Fig. 8 the cable .leading from the batteries in the compartment 65 may be DIOVided at. its H end with a plug receptacle generally indicated by the numeral 56 for electrically conn'ectingthe cable leading from the batteries to a terminal plug 61 carried upon the panel 48;

The operation of the instrument ,is as :01

lows-First the oscillator tube is,.turned on, which same may be accomplished through the medium'of the switch 68, after which a tuning fork of predetermined pitch is set in vibration and held adjacent the poles of the magnetic pickup H. At this time both the-- tone of the oscillator and that of the tuning'fork will be heard simultaneously'by'the operator by means of the earphones 53. If the two tones are not in resonance, that is, do not coincide fin frequency, beats will be heard. The dial 69 of the r tuning condenser 63 is turned, adjusting the frequency of oscillation of the. oscillator tube 58 until. the beats are eliminated, thus indicating that the tuning fork'and oscillator tube are in resonance as to frequency. An adjustablezero pointer iii is provided in connection with the dial 69. which may, when resonance is obtained, be

set at the zero indication on the dial 69. Thus 7 if the dial is inadvertently moved from its original position, such error may bereadily corrected. After ,the oscillator tube isadjusted to emit a musical note the frequency of which is known, thewire .a, or other'object under testis set in vibration by striking it, with a mallet and the tone emitted by the wire is compared with the tone emitted by the oscillator tube by listening to the two tones alternately or simultaneously.

through the earphones. If the two tones are of decidedly difierent pitch, the difference will be easily detected. Under these circumstances the movable clampingmember It may then be shifted in such direction as to bringthe pitch of the two tones closer together. This may be readily accomplished by turning the hand wheel 34 manually either clockwise or counter-clockwise to cause the roller to rotate and move theclamping member I! either up or down, consequently increasing or decreasing the vibrating length of the wire between. the primary fixed clamping mem- ,two notes, the needle of her II and secondary or movable clamping member il. when the two tones are nearly alike in pitch. beats will be heard and as the movable clamping member is furtheradjusted, the beats will be entirely eliminated. At this time with the pointer adjusted in the manner heretofore described toindicate on the correct scale. the stress or lo'adin the member under test may be read directly from the scale. 7 i a when the wire or other object being. tested is of nonemagnetic material, I have provided a nonmagnetic adapter H, as shown in detail in Fig. 'l,

n made ofmagnetic material which may be readily leading to the secondary windings ll heretofore mentioned. Thus an alternating E. M. F. is

. be-connected in series with the output of the audio amplifying circuit, which may be turned on or oil by the switch.' -In this manner a visual indication will be obtained of the-difference between the note produced in the oscillator tube and that producedin the electromagnetic pickup. That is to say, as long as a difference exists between the v the output meter will gradually move either to right or left. When the beats produced by the notes are entirely eliminated, the needle will become stationary, and indicatethat resonance has been attained.

Fig. 10 illustrates a still further modification of I the wiring diagram illustratedin Fig. 8. In this modification of the invention the output from the audio amplifying circuit-has connected thereto an oscillograph l4 capableof being turned on or ,7 off by the switch 15. In this manner a perma-' of the notes produced may be ob- -nent record tained.

I claim: r I 1. In a measuring instrument, means for producing a constant magnetic field, a vibrating ele;

ment of flxed'tension associated with said mag- 'netic field to produce an alternating E. M. R, a. detector circuit for translating said alternating f E. M. F. into a musical: note, an oscillatory circuit associated with said detector circuit for emit-' ting a second musical note, and means for tuning [one of said circuits to bring the frequencies of the notes of both circuits into resonance with respect to one another.

2. In a measuring instrument, means for pro-- ducing a constant magnetic field, a vibrating element of fixed tension. associated with said magnetic field to produce an alternating E. M. F., a

detector circuit for translating said alternating E. M. F. into a. musical note, an oscillatory circuit associated with said detector circuit for emitting 1 a second musical note, and means for tuning said oscillatory circuit to bring the note emitted'therefrom into resonance with saidfirst mentioned 3. In a measuring instrument, means for producing a constantqinagnetic field, a vibrating element v of fixed tension associated with said constant magnetic field to produce an oscillating E. M. F., a detector circuit for translating said alternating E. M. F. into ,amusical note, an oscillatory circuit associated with said detector circuit for emitting a second musical note, means for tuning one ofsaid'circuits to bring the frequen-f cies of the notes of both" circuits into resonance. 1 and means associated with-said: last mentioned means for indicating said frequency. I 4. In a measuring instrument, means for producing a constant magnetic field, a vibrating element of fixed tension associated with said constant" magnetic field to produce an alternating I. I. l"., a detector circuit for translating said alternating E. M. 1''. into a musical'note, an oscila latory circuit associated with said detector'circuit, means for tuning said oscillatory circuit to emit a second musical note of a desired known frequency and means for tuning said detector circuit to 19; bring its note'into resonance with said last m tioned note. g

'5. In a measuring instrument, means for producing a constant magnetic field, a vibrating ele-' ment of fixed tension associated with said constant magnetic field to produce an alternating E. M. 1"., a detector circuit for translating said alternating E. M. 1". into a-musical note, an oscillatory circuit associated with said detector circuit,

meam for tuning said oscillatory circuit to emit a second musical note of a desired known frequency, means for tuning said detector circuit to bring its, note into resonancewith said last men tioned note and means associated with said oscillatory circuittuning means for indicating the frequency of its emitted note.

6. In a measuring instrument, means for producing a constant magnetic field, a vibrating element of fixed tension, means for mounting said vibrating element adjaunt said constant mag netic field; means to produce an alternating E. M, It, a detector circuit for translating said alternating .13. NJ". into a musical-note, an oscillatory circuit coupled to said detector circuit,

a means for tuning said oscillator! circuit to emit 'a second. musical note of a desired known frequency and means fortuning said detector circuit to bring its note'to resonancewith said last mentionednote. i

'l. In a measuring instrument, means for producing a constant magnetic field, avvibrating element of fixed tension associated with said con stant magnetic field "to produce an alternating I. I. 1"., a detector circuitfor translating said alternatingE. M. 1''. into a current of'one fre-' uency; ail-oscillatory circuit associated with said detector circuit for emitting a second-currenthaving a different frequency, means for tuning one of said circuits to'bring the frequency of its current into with the frequency of the other current and visual means common to said circuits for indicating when the frequencyof said currents are identical.

8. In ameasuring 'ent, mea'ns for producing a constant-magnetic field. a. vibrating-elementof fixed tension associated with said constant magnetic field-toinroduce an alternating I. M. l, a detector circuit for translating said alternatingl. M. 1''. into a current of one frequency. an oscillatory circuit associated 'with. said detector circuit-for emitting a second current having a diiIerent frequency, 'means for'tuning said oscillatory circuit to bring the frequency of its current into resonance with" the frequency'of the other current and-visual means common, to said circuits for; indicating when the frequency ofsaidcurrentsareidenticaly f v 9. In a measuring instrument, means for-pro? ducing a constant magnetic field. a'vibrating element-of fixed tension associated with said constant magnetic field to producean alternating I. M. 1".."a detector circuitf or said alternating Ii. M. I". intoa current of one frequency. an oscillatory circuit associated with said detectoricircuit for emitting a .second current having a different frequency, means for .tuning said detector circuit ,to bring the'fr equency of its current into resonancewith the frequency of thecurrent emitted by said oscillatory circuit and visual means commonto said circuits for indi- I catin'g when thefrequenciea of saidlcurrentsare identical.

10. In a measuring instrument, for

ducing a constant magnetic field, a vibrating element of fixed tension associated with said constant magnetic field to produce an alternating I'M. It, adetector circuit fortranslatingsaid alternating E. M. 1''. into a current of unknown "frequency, an oscillatory circuit associated with said detector circuit, means fortuning said oscillatory circuit to emit a second current of a fixed.- known frequency, means for tuningsaid detector circuit to bring the frequency, of itscurrent into resonance withthe current of known frequency and visual means common to saidcircuits for indicating when the frequency oi said currents are identical. a

vi1. In a measuring instrument, means forproducing aconstant magnetic field. a vibrating'eleq ment offixed tension associated with said constant magnetic field to produce an alternating E. M. It, a detector circuitfor translating said alternating EM. 1''. into a current of unknown frequency, an oscillatory circuit associated with said detector circuit, means including a variable condenser for tuning said-oscillatory circuit to' emita second current of a fixed knownfrequency,

the frequency of its current into resonance with the current of'known frequency, visual means common tosaid circuits for indicating when the frequency of. said currents areidentical and further means cooperating with'said variable condenser for indicating directly the frequency of detector circuit, variable -means for tuning said oscillatory circuit to emit a second musical note of ;a desired known frequency, said mounting -means including a manually controlled mechanism for changing the irequency of vibration of [said vibrating element whereby the musical note of said detector circuit is caused to correspond with the second musical note of known frequency.

-' 13. In a measuring instrument. means for pro-' 7 ducing a constant magnetic field. a vibrating element of fixed tension, means for mounting said vibrating element for vibration in said constant magnetic field toproduce an alternating E. M. l"., a detector circuit for translating said alternating E. M. I". intoamusical note'of unknown fm quencylan oscillatory circuit associated with saiddetector circuit, variable means for tuning said oscillatory circuit to emit a second musical note of adesired known frequency, said mounting 'means including a manually controlled mechanism for varying the effective vibrating length of said vibrating element to change the frequencies of vibration of the musical note translated by said detector circuit and produce a note corresponding to the musical note of known frequency.

- 14. In a measuring instrument, means for producing a constant'magnetic field, a vibrating element, under a fixed tension, means iorv mounting means for tuning said detectorcircuit to bring a said vibratingeiement forvibration in said stant magnetic field to produce an alternating E. M. l"., a detector circuit for translating said alternating E. M. F. into a musical'note offun known frequency, an oscillatory circuit associated with said detector circuit, variable means for tuning said oscillatory circuit to emit a second musical note of a desired known frequency, said mounting means including a manually'controlled mechanism for the effective vibrating length of said vibratingelement to change the frequency of vibration of the musical note translated by said detector circuit and produce a note corresponding to the musical note of known fre quency and calibrated means for indicating the tension in said vibrating element when said mu- I sical notes are in resonance.

15. In a measuring instrument, means for pro-Y ducing a constant magnetic field, avibrating ele-' ment under a fixed tension, means for mounting said vibrating element for vibration in said con-- stant magnetic field to produce an alternating E. M. F a detector circuit for translating said. alternatingE. M. F. into a musical note of unknown frequency, an oscillatory circuit associated with said detector. circuit, variable means for" tuning said oscillatory circuit to emit a second musical note of a desired known frequency,sa.id

501 1115111 for changing the frequency of vibration of mounting means including a manually controlled mechanism for changing the frequency of vibration of said vibrating elements, said means com prising a fixed clamping member and a movable clamping member for varying the effective vibrating length of said vibrating element whereby the musical note translated by said detector circuit is caused to correspond to the musical'note of known-frequency. L,

16. In a measuring instrument, means for-producing a constant magnetic field, a vibrating element under .a fixed tension, means for mounting said vibrating element forvibration in said constant magnetic field. to produce an alternating E. M. F., a'detector circuit for translating said alternating E. M. F. into a current of unknown frequency, an oscillatory circuit associated with said detector circuit, variable means for tuning said oscillatory circuit to emit a second current of a desired known frequency, said mounting means including a manually controlled mechasaid vibrating element comprising a fixed clamping member and a movable clamping'member for varying the eifective vibrating length of said vibrating element, whereby'the frequency of the- "current translated by said detector'circuit is caused to correspond to the known current frequency and visual means for indicating when the frequencies of said currents are identical.

17. In a measuring instrument, means for producing a constant magnetic field, a vibrating ele-' ment under a fixed tension, means formounting said vibrating element for vibration in said constant magnetic field to produce an alternating E. M. -F., a detector circuit for translating said alternating E. M. F. into a current of unknown frequency, an oscillator! circuit associatedwith said ,detector circuit, variable means for tuning said oscillatory circuit to emit a second current of a desired known frequency, said mounting means including a manually controlled mechanismfor changing the frequency of vibration of said vibrating element comprising a fixed clamping member and a movable clamping member for varying the effective vibrating length of said vibrating element whereby the frequency of the concurrent 'translated by said detector circuit is caused to correspond to the known current frequency, means, associated with said movable clamping member for indicating directly the tension in said vibrating element and visual means for indicating when the frequencies of said currents are identical. a

18. In a measuring instrument, means for producing '-a constant magnetic fields vibrating ele-.

ment under a fixed tension, means for mounting said vibrating elementfor stant magnetic field to produce an alternating 1o vibration in saidcon- E M. F., a' detector circuit for translating said alternating F. into a musical note of 'un-.

known frequency, an oscillatory circuit associated with said detector circuit, variable means forv tuning said oscillatory circuitv to emit asecond musical note of a. desired known frequency, said mounting means including a manually controlled mechanism for changing the frequencyof vibrati'on of said vibrating element comprising a fixed clamping member and a movable clamping member for varying the effective vibrating length of said vibrating" element whereby the musical note translated by said detector circuit is caused 1 to correspond to the musical note of known frequency, visual means for indicating when the fre- .quencies of said notes are identical and further 'means associated with 'said movable clamping.

member for indicating directly the tension said vibrating element, said last mentioned means comprising a pointer carried by said movable clamping member and a calibrated scale cooperating with said pointer for indicating said ten- I sion.

19. In-a measuring instrument, means for producing a constant magnetic field, a vibrating element under a fixed tension, means for mounting said vibrating element for vibration stant magnetic'field to produce an E. M. F.,

in said conalternating a detector circuitfor translating said alternating E. M. F. into a musical note of unknown frequency, an oscillatory circuit associated with said detector circuit, variable means for tuning said oscillatory circuit to emit a second musical note of a desired known frequency,

said mounting means comprising a support, a,

manually controlled mechanism on said support for changing the frequencies of vibration of said vibrating element comprising a fixed clamping member and a movable clamping member for varying the respective vibrating length of said vibrating element whereby the musical note translated by said detector circuit is caused to correspond to the musical note of known frequency,

and means associated with said movable clamping tension in 'in a. vibratory structure under a given tension comprising a support, a plurality of clamping,

members on said support for clamping therebetween variable lengths ofthe vibratory structure to vary the natural frequency of vibration, electro-magnetic means carried by said support and responsive to the vibration of said vibratory. structure for translating the frequency of vibration of said vibratory structure into amusical 'note and means associated with one of said in said vibratory structure when "said structure produces a note oi predetermined frequency.

21. A testing device for determining the stress ina' vibratory structure'under' agiven tension comprising a support, clamping means onsaid support ior clamping therebetween variable lengths or the vibratory structure to vary vits natural irequency oi vibration comprising a fixed clamping member and a movable clamping member, .electro-magnetic'means carried by said sup port and responsive to the vibration oi said vibratory structure ior translating the irequency oi vibration oi said strucmre into ,a musical note and means associated with said movable clamping member ior. indicating directly the stress in said vibratory structure when the latter produces a note oi predetermined irequency. g,

21A testing device ior'determining the stress in'a vibratory structure under a given tension a support, a plurality oi clamping means on said support ior clamping therebetween variable lengths oi the vibratory structure to vary its natural irequency oi vibration comprising a fixed clamping, member and a movable clamping member, electromagnetic means carried by said support and responsive tothevibration of said vibratory structure ior translating the irequency oi vibration oi said structure into a musical note, and means associated with said movable clamping member ior indicating directoi said vibratory structure into a. musical, note,

ly the stress in said the latter produces. a note oi predetermined ire. quency. said last mentioned means comprising,- a pointer carried by saidmovable clamping member and a calibrated scale on said support over which said pointer operates ior indicating directly strqs in said vibratory structure when the latter produces a musical, note oi a predetermined irequency. a

23. A testing device ior determining the stress in a vibratory structure under a given tension comprising a support, clamping means-on said support ior clamping therebetween 'rvariable lengths oi the vibratory structure to vary its natural frequency oi vibration comprisinga fixed clamping member and a-movable clamping member, electromagnetic carried by said support and responsive to the vibration oi-said vibratory structure ior translating the vibrations means associated with said movable clamping member ior indicating directly the stress ,in the structure being tested when the latter produces a note oi predetermined irequency, said means comprising aplurality oi scales calibrated in 2' terms oi tension ior vibratory structuresoi diiierent mass and elasticity and a pointer carried by said movable clamping member and adjustable relativethereto'to indicate on any one oi .said

scales.

MENDEL N. PACK. 

